The field of the invention is nuclear magnetic resonance imaging methods and systems. More particularly, the invention relates to the in vivo imaging of tissue temperature.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, Mz, may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited spins after the excitation signal B1 is terminated, this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (Gx, Gy and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
Hyperthermia has been shown to be highly valuable as an adjunct to radiation therapy in such cases as recurrent cancer in the chest wall. One of the key factors in successful hyperthermia treatment is the measurement and control of temperature in the tumor and also in surrounding normal tissue. While invasive thermometry provides accurate and precise measurements, complete temperature mapping of a region using magnetic resonance imaging is expected to afford improvements in the control of the temperature therapy distribution. Non-invasive thermometry is needed for radiofrequency ablation to heat tumors, for cryoablation to freeze tumors and to provide temperature measurements within the tumor as well as the surrounding tissues.
Previous work has shown the value of using the temperature sensitivity of the tissue water proton resonant frequency shift (PUS) or the apparent diffusion coefficient (ADC) to measure temperature change. However, tissues containing a mix of water and lipids, e.g. breast, confound most standard frequency shift thermometry approaches since lipids have no chemical shift dependencies with temperature change.
Recently, a new method known as IDEAL was developed for imaging spin species such as fat and water. As described in U.S. Pat. No. 6,856,134 B1 issued on Feb. 15, 2005 and entitled “Magnetic Resonance Imaging With Fat-Water Signal Separation”, the IDEAL method employs pulse sequences to acquire multiple images at different echo times (TE) and an iterative, linear least squares approach to estimate the separate water and fat signal components. The advantage of the IDEAL method is if the frequencies of the particular metabolites being imaged are known, the number of different echo time repetitions can be significantly reduced. This “a priori” information shortens scan time and enables more pulse sequence repetitions to be devoted to increased image resolution.